It is well known to provide a coin validator which has a coin sensor for testing coins by subjecting them to an electromagnetic field and determining the effect of the coin on the field, for example, by measuring the change in the amplitude and/or frequency of the sensor output. Some sensors are used to produce a result which is predominantly dependent upon the coin thickness. The sensor output will also be influenced by other factors, but this does not affect the ability of the sensor to distinguish between coins of different types and therefore other such influences will be ignored for the purposes of the present specification.
A thickness sensor may consist of a coil driven at a relatively high frequency and positioned at one side of the coin path. As the coin passes, it travels along a ramp which is so inclined that the coin face tends to lie against a reference plane as the coin passes the sensor. The coil is located on the other side of the passageway, and its field will be influenced to an extent depending on the location of the coin's closest face with respect to the coil, which in turn will depend on coin thickness. However, in spite of the inclination of the ramp, the coin flight tends to be fairly unstable and therefore errors are encountered due to the possible variations in the coin position with respect to the coil.
In order to mitigate such problems, some thickness sensors are formed with two coils connected together in series or parallel. A first one of the coils is close to the reference plane and the other is at the opposite side of the passageway. For a coin of a particular thickness, any reduction in the effect of the coin on the first coil's field caused by the coin being spaced away from the reference plane is substantially compensated by an increase in the effect of the coin on the other coil's field. However, this is only effective for variations close to a so-called "balance point", and therefore such improvements tend to apply only to a fairly small range of coin thicknesses, and a small range of coin positions with respect to the reference plane.
Attempts have been made to stabilise the coin flight, using for example snubbers for absorbing the coins' kinetic energy, but the problem still remains. Furthermore, it is becoming more desirable to reduce the size of coin validators, and this tends to increase the coin flight instability in that it reduces the distance in which the coins' flight might be stabilized prior to reaching the sensors.